How to Use Monolog to Write Logs

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How to Use Monolog to Write Logs

Monolog is a logging library for PHP used by Symfony. It is inspired by the Python LogBook library.

Usage

To log a message simply get the logger service from the container in your controller:

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public function indexAction()
{
    $logger = $this->get('logger');
    $logger->info('I just got the logger');
    $logger->error('An error occurred');

    // ...
}

The logger service has different methods for different logging levels. See LoggerInterface for details on which methods are available.

Handlers and Channels: Writing Logs to different Locations

In Monolog each logger defines a logging channel, which organizes your log messages into different "categories". Then, each channel has a stack of handlers to write the logs (the handlers can be shared).

Tip

When injecting the logger in a service you can use a custom channel control which "channel" the logger will log to.

The basic handler is the StreamHandler which writes logs in a stream (by default in the app/logs/prod.log in the prod environment and app/logs/dev.log in the dev environment).

Monolog comes also with a powerful built-in handler for the logging in prod environment: FingersCrossedHandler. It allows you to store the messages in a buffer and to log them only if a message reaches the action level (error in the configuration provided in the Symfony Standard Edition) by forwarding the messages to another handler.

Using several Handlers

The logger uses a stack of handlers which are called successively. This allows you to log the messages in several ways easily.

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# app/config/config.yml
monolog:
    handlers:
        applog:
            type: stream
            path: /var/log/symfony.log
            level: error
        main:
            type: fingers_crossed
            action_level: warning
            handler: file
        file:
            type: stream
            level: debug
        syslog:
            type: syslog
            level: error

The above configuration defines a stack of handlers which will be called in the order they are defined.

Tip

The handler named "file" will not be included in the stack itself as it is used as a nested handler of the fingers_crossed handler.

Note

If you want to change the config of MonologBundle in another config file you need to redefine the whole stack. It cannot be merged because the order matters and a merge does not allow to control the order.

Changing the Formatter

The handler uses a Formatter to format the record before logging it. All Monolog handlers use an instance of Monolog\Formatter\LineFormatter by default but you can replace it easily. Your formatter must implement Monolog\Formatter\FormatterInterface.

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# app/config/config.yml
services:
    my_formatter:
        class: Monolog\Formatter\JsonFormatter
monolog:
    handlers:
        file:
            type: stream
            level: debug
            formatter: my_formatter

How to Rotate your Log Files

Over time, log files can grow to be huge, both while developing and on production. One best-practice solution is to use a tool like the logrotate Linux command to rotate log files before they become too large.

Another option is to have Monolog rotate the files for you by using the rotating_file handler. This handler creates a new log file every day and can also remove old files automatically. To use it, just set the type option of your handler to rotating_file:

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# app/config/config_dev.yml
monolog:
    handlers:
        main:
            type:  rotating_file
            path:  '%kernel.logs_dir%/%kernel.environment%.log'
            level: debug
            # max number of log files to keep
            # defaults to zero, which means infinite files
            max_files: 10

How to Disable Microseconds Precision

2.11

The use_microseconds option was introduced in MonologBundle 2.11.

Setting the parameter use_microseconds to false forces the logger to reduce the precision in the datetime field of the log messages from microsecond to second, avoiding a call to the microtime(true) function and the subsequent parsing. Disabling the use of microseconds can provide a small performance gain speeding up the log generation. This is recommended for systems that generate a large number of log events.

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# app/config/config.yml
monolog:
    use_microseconds: false
    handlers:
        applog:
            type: stream
            path: /var/log/symfony.log
            level: error

Adding some extra Data in the Log Messages

Monolog allows you to process the record before logging it to add some extra data. A processor can be applied for the whole handler stack or only for a specific handler.

A processor is simply a callable receiving the record as its first argument. Processors are configured using the monolog.processor DIC tag. See the reference about it.

Adding a Session/Request Token

Sometimes it is hard to tell which entries in the log belong to which session and/or request. The following example will add a unique token for each request using a processor.

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namespace AppBundle;

use Symfony\Component\HttpFoundation\Session\Session;

class SessionRequestProcessor
{
    private $session;
    private $token;

    public function __construct(Session $session)
    {
        $this->session = $session;
    }

    public function processRecord(array $record)
    {
        if (null === $this->token) {
            try {
                $this->token = substr($this->session->getId(), 0, 8);
            } catch (\RuntimeException $e) {
                $this->token = '????????';
            }
            $this->token .= '-' . substr(uniqid(), -8);
        }
        $record['extra']['token'] = $this->token;

        return $record;
    }
}
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# app/config/config.yml
services:
    monolog.formatter.session_request:
        class: Monolog\Formatter\LineFormatter
        arguments:
            - "[%%datetime%%] [%%extra.token%%] %%channel%%.%%level_name%%: %%message%% %%context%% %%extra%%\n"

    monolog.processor.session_request:
        class: AppBundle\SessionRequestProcessor
        arguments:  ['@session']
        tags:
            - { name: monolog.processor, method: processRecord }

monolog:
    handlers:
        main:
            type: stream
            path: '%kernel.logs_dir%/%kernel.environment%.log'
            level: debug
            formatter: monolog.formatter.session_request

Note

If you use several handlers, you can also register a processor at the handler level or at the channel level instead of registering it globally (see the following sections).

Registering Processors per Handler

You can register a processor per handler using the handler option of the monolog.processor tag:

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# app/config/config.yml
services:
    monolog.processor.session_request:
        class: AppBundle\SessionRequestProcessor
        arguments:  ['@session']
        tags:
            - { name: monolog.processor, method: processRecord, handler: main }

Registering Processors per Channel

You can register a processor per channel using the channel option of the monolog.processor tag:

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# app/config/config.yml
services:
    monolog.processor.session_request:
        class: AppBundle\SessionRequestProcessor
        arguments:  ['@session']
        tags:
            - { name: monolog.processor, method: processRecord, channel: main }
This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.
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